As Claudia Hartley packed away the instruments she used to operate on a blind Bornean orangutan named Aan, she had every reason to believe that the surgery had been successful and the red-haired primate would see again.

Aan had lived in an enclosure on a wildlife reserve since she strayed onto a palm oil plantation in 2012 and was shot with more than 100 air rifle pellets.

Blinded in one eye and severely sight-impaired in the other, she would not have survived in the wild.

Hartley, who travelled to Borneo from Cambridge, hoped the orangutan would be able to return to the rainforest if the cataracts were removed from her right eye.

But a few days after the surgery in February this year, it was clear that Aan could still not see.